For example, a directional coupler described in Japanese Unexamined Patent Publication No. 2013-5076 is known in the related art. The directional coupler described in Japanese Unexamined Patent Publication No. 2013-5076 includes first to fourth terminals, a main line that is connected between the first terminal and the second terminal, a first sub line that is connected to the third terminal and is electromagnetically coupled to the main line, a second sub line that is connected to the fourth terminal and is electromagnetically coupled to the main line, and a phase conversion unit that is connected between the first sub line and the second sub line and causes a phase difference in a passing signal. In the directional coupler, the main line, the first sub line, and the second sub line are disposed between a pair of ground layers which are connected to the ground.